One of my sapphire 5830's does this, the reason being too low fan speed. Core stays at a stable 55-60c but the VRMs are overheating due to a lack of airflow to them (fan speed is about 35%). It crashes when the VRM gets too hot.

Also, even relatively low overclocks or voltage increases can cause these cards to crash after a few hours.

Crappy design but you get what you pay for....